Preparation and characterization of tantalum/polyaniline composite based chemiresistor type sensor for hydrogen gas sensing application

In the present work we have reported the effect of Shift heavy ion (SHI) irradiation on the gas sensing properties of tantalum (Ta)/Polyaniline (PANI) composite thin film based chemiresistor type gas sensor for hydrogen gas sensing application. PANI was synthesized chemically by in situ oxidative polymerization method. The thin sensing films of PANI were deposited onto finger type Cu-interdigited electrodes using spin cast technique and a thin Ta layer was deposited on to PANI thin film to prepare Ta/PANI composite chemiresistor sensor. These chemiresistor sensing films were irradiated with energetic Au⁺¹² ions (150 MeV) at the different fluencies ranging from 1 × 10⁹ to 1 × 10¹¹ ions/cm². The structural and morphological properties of these composite thin films were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements before and after SHI irradiation. The electrical properties of these composite thin films were characterized by I–V characteristic measurements. The changes in resistance of the composite thin film sensor were utilized for detection of hydrogen gas. It was observed that after SHI irradiation Ta/PANI composite sensor shows a high response value and sensitivity with good repeatability in comparison to the pristine sample.     

Synthesis and characterization of TiO2 doped polyaniline composites for hydrogen gas sensing

The Polyaniline (PANI) and Titanium dioxide (TiO₂)/PANI composite thin film based chemiresistor type gas sensors for hydrogen (H₂) gas sensing application are presented in this paper. Pure PANI and TiO₂/PANI composites with different wt% of TiO₂ were synthesized by chemical oxidative polymerization of aniline using ammonium persulfate in acidic medium at 0-5 °C. Thin films of PANI and TiO₂/PANI composites were deposited on copper (Cu) interdigited electrodes (IDE) by spin coating method to prepare the chemiresistor sensor. Finally, the response of these chemiresistor sensors for H₂ gas was evaluated by monitoring the change in electrical resistance at room temperature. It was observed that the TiO₂/PANI composite thin film based chemiresistor sensors show a higher response as compared to pure PANI sensor. The structural and optical properties of these composite films have been characterized by X-ray diffraction (XRD) and UV-Visible (UV-Vis) spectroscopy respectively. Morphological and structural properties of these composites have also been characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) respectively.     

Hydrogen sensor using the Pd film supported on anodic aluminum oxide

Highly sensitive hydrogen gas sensors were fabricated using a microelectromechanical system (MEMS) and anodic aluminum oxides (AAOs) process. MEMS based gas sensor platform was designed with the multi-layer type for Pd film morphology manipulations. The operating temperature of the micro heater was positively correlated with the heater. Hydrogen sensing response of the sensor showed a good positive linearity as the gas sensitivity increased with increasing hydrogen concentration. The hydrogen sensitivity (defined as ratio of sensor resistances in air and after the hydrogen gas injection) was 0.638% at hydrogen concentration of 2000 ppm. The H₂ sensitivity was very dependent on the thickness and morphology of Pd-nanosized film. The gas sensitivity and response properties showed different behaviors when palladium film was deposited on the anodic aluminum oxide (AAO) layer. The hydrogen sensitivity for the Pd on AAO layer was about 0.783% at the hydrogen concentration of 2000 ppm. The sensitivity of the Pd-AAO layer improved with respect to the pure Pd thin film due to nanoporous nature of AAO.     

Thin film Pt/TiO2 catalysts for the polymer electrolyte fuel cell

Thin film Pt/TiO₂ catalysts are evaluated in a polymer electrolyte electrochemical cell. Individual thin films of Pt and TiO₂, and bilayers of them, were deposited directly on Nafion membranes by thermal evaporation with varying deposition order and thickness (Pt loadings of 3–6 μg cm⁻²). Structural and chemical characterization was performed by transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Oxygen reduction reaction (ORR) polarization plots show that the presence of a thin TiO₂ layer between the platinum and the Nafion increases the performance compared to a Pt film deposited directly on Nafion. Based on the TEM analysis, we attribute this improvement to a better dispersion of Pt on TiO₂ compared to on Nafion and in addition, substantial proton conduction through the thin TiO₂ layer. It is also shown that deposition order and the film thickness affects the performance.     

Impedance analysis of PLD LiNi0.8Co0.2O2 film electrode

The kinetics of intercalation are discussed using a pulsed laser deposition (PLD) film electrode and electrochemical impedance analysis. Films of LiNi_0.8Co_0.2O₂, deposited on single crystal substrates, were used for the study. The films have intercalative or blocking orientations on different crystal surfaces of the substrates. Impedance spectra show that there are at least three elemental processes in intercalation. Two processes at higher frequencies suggest that they occur at the electrode surface and are influenced by the orientation of the film. The third process appearing at low frequencies below 1 Hz indicates lithium motion in the bulk structure and shows the largest resistance among the three processes. This lithium conduction in a thin PLD film shows a semicircular response and is considered to be influenced more by the structure due to the nanometer-scale thickness.     

An optical hydrogen sensor based on a Pd-capped Mg thin film wedge

We report the proof of concept of a thin film device with a one-to-one relationship between the H₂ partial pressure and the lateral progression of a visible optical change along a thin film multilayer 70 mm long. The device basically consists of a sensing Mg layer with a thickness gradient. It exploits the thickness dependence of the hydrogenation thermodynamics of Pd-capped Mg thin films. The optical change of the Mg layer during the metal–hydride transition can be detected both in reflection and in transmission. This optical sensor allows a continuous measurement of hydrogen partial pressure in the range between 200 and 4000 Pa.     

Hysteresis behavior of electrical resistance in Pd thin films during the process of absorption and desorption of hydrogen gas

We report the fabrication of a novel hydrogen sensor that utilizes the electrical resistance changes in the palladium thin films with nanometer thicknesses. The sensing mechanism is based on transitory absorption of hydrogen atoms into the palladium layer, which leads to the reversible alteration of the electrical resistance. In concentrated hydrogen ambient, the excess hydrogen absorption process leads to mechanical deformation on the surface of the palladium films, corresponding to the phase transition from α-phase to β-phase. The reversible sensing process results in a hysteresis curve for resistive properties, of which the height (sensitivity) could be controlled by manipulating the thickness of the palladium layers. The peel-off phenomena on the surface of the palladium film were suppressed by decreasing the thickness of the film. At the thickness of 20 nm, a hysteresis curve of resistance was obtained without any structural change in the palladium thin film. These results provide a significant insight to the fundamental understanding of the relationship between the electrical sensitivity of pure Pd thin films and related structural deformation, which is essential to develop robust H-sensors with high sensibility.     

Cathodic electrodeposition of Cu2S thin film for solar energy conversion

Cathodic electrodeposition in the presence of EDTA in aqueous solution was used to prepare Cu₂S thin film deposited on Ti substrate. The effect of deposition potential, concentration and deposition time was studied to determine the optimum condition for electro-deposition process. Cyclic voltammetry was performed to elucidate the electrodic processes that occur while potentials for electrodeposition were applied to determine the optimum potential for electrodeposition. The thin films are characterised by X-ray diffractometry. The photoactivity of the deposited films and their conduction types were evaluated using photoelectrochemical technique. The band gap energy and type of optical transitions were determined from optical absorbance data.     

Hydrogen gas sensing based on polyaniline/anatase titania nanocomposite

Polyaniline (emeraldine)/anatase TiO₂ nanocomposite (PA-NC) was prepared by a chemical oxidative polymerization. The thin films of PA-NC for hydrogen gas sensing application were deposited on Cu-interdigited electrodes by spin coating technique. A study on characteristics of PA-NC thin films was demonstrated by a porous cylindrical morphology. The response and response/recovery time of sensors for hydrogen gas were evaluated by the change of TiO₂ wt% at environmental conditions. Resistance-sensing measurement was exhibited a high sensitivity about 1.63, a good Long-term response, low response time and recovery time about 83 s and 130 s, respectively, at 0.8 vol% hydrogen gas for PA-NC including 25% wt of anatase nanoparticles. The current–voltage characteristics of PA-NC gas sensors before and after hydrogen gas injection showed a nonlinear ohmic current. Moreover, we studied the formation of PA-NCs and their hydrogen gas sensing mechanism based on contact regions in PA-NC.     

Production of capacitive films from Mn thin films: Effects of current density and film thickness

Electrochemical oxidation of Mn thin films produces a highly capacitive, porous MnO₂ surface layer. The effects of current density and deposited Mn layer thickness on the morphology of the porous surface layer are quite pronounced. A higher current density results in a much thinner, finer porous layer while thicker deposited Mn films give much thicker porous films. Increasing the current density results in a film with greater hydration and an increase in capacitance. For the films of varying deposited layer thickness, oxidation occurs at a single current density and, as a result, the relative hydration of the film does not change noticeably. Increasing the deposited layer thickness results in a porous surface layer that increases in thickness, but with a constant amount of hydration. This combination of trends results in a significant increase in the areal capacitance of the film but little change in the specific capacitance.     

Physically and chemically synthesized TiO2 composite thin films for hydrogen production by photocatalytic water splitting

TiO₂ thin films have been synthesized by radio-frequency magnetron sputtering and sol–gel method to study the hydrogen generation by photocatalytic water splitting under visible light irradiation. Photoelectrochemical cell with chemical bias, involving photo-anode in form of TiO₂ film deposited on conducting indium tin oxide (ITO) film and Pt as cathode, is developed. The effect of conducting ITO layer on photo-voltage is studied by varying the thickness of ITO films. Constant H₂ generation rate is obtained for long period of time by both the TiO₂ films because of the separated evolution of H₂ and O₂ gas, thus eliminating the back-reaction effect. Sputter-deposited film as compared to sol–gel-synthesized film showed better H₂ generation rate, mainly explained in terms of the higher visible light absorption achieved by oxygen vacancies created in the TiO₂ film by the energetic target ions during deposition in pure Ar gas pressure.     

Preparation and characterization of RuO2 thin films

We report on the preparation and characterization of RuO₂ thin films by metal-organic chemical vapor deposition (MOCVD) method and reactive sputtering under various conditions. The Auger electron spectroscopy depth profile shows good compositional uniformity across the thickness of the films. As confirmed by X-ray investigations, the films crystallize with the correct rutile structure. The results of the electrical and optical studies of the films show a metallic character of the films deposited at substrate temperature higher than 100°C. The grain-boundary scattering model fits well for the films with an average grain size of about 12–50 nm. The red shift and broadening of the line width of the Raman peaks are analyzed by the spatial correlation model. The results of Raman investigation indicate that a nearly strain free and high-quality RuO₂ thin film could be deposited on a Si substrate.     

Total SiH4/H2 pressure effect on microcrystalline silicon thin films growth and structure

The effect of the total SiH₄/H₂ gas pressure (1–10 Torr) on the growth rate, the film crystallinity and the nature of hydrogen bonding of microcrystalline silicon thin films deposited by 13.56 MHz plasma-enhanced chemical vapour deposition (PECVD) was investigated under well-controlled discharge conditions. The deposition rate presents an optimum for 2.5 Torr, which does not follow the trend of silane consumption that increases with pressure and is attributed to an increase in plasma density. The film crystallinity increases with pressure from 1–2.5 Torr and then remains almost the same, whereas the films deposited at 1 Torr are highly stressed. On the other hand, hydrogen bonding is also drastically affected.     

p-Type CuSbS2 thin films by thermal diffusion of copper into Sb2S3

We report the preparation of copper antimony sulfide (CuSbS₂) thin films by heating Sb₂S₃/Cu multilayer in vacuum. Sb₂S₃ thin film was prepared from a chemical bath containing SbCl₃ and Na₂S₂O₃ salts at room temperature (27 °C) on well cleaned glass substrates. A copper thin film was deposited on Sb₂S₃ film by thermal evaporation and Sb₂S₃/Cu layers were subjected to annealing at different conditions. Structure, morphology, optical and electrical properties of the thin films formed by varying Cu layer thickness and heating conditions were analyzed using different characterization techniques. XRD analysis showed that the thin films formed at 300 and 380 °C consist of CuSbS₂ with chalcostibite structure. These thin films showed p-type conductivity and the conductivity value increased with increase in copper content. The optical band gap of CuSbS₂ was evaluated as nearly 1.5 eV.     

Preparation and electrochemical properties of Li4Ti5O12 thin film electrodes by pulsed laser deposition

Spinel Li₄Ti₅O₁₂ thin film anode material for lithium-ion batteries is prepared by pulsed laser deposition. Thin film anodes are deposited at ambient temperature, then annealed at three different temperatures under an argon gas flow and the influence of annealing temperatures on their electrochemical performances is studied. The microstructure and morphology of the films are characterized by XRD, SEM and AFM. Electrochemical properties of the films are evaluated by using galvanostatic discharge/charge tests, cyclic voltammetry and a.c. impedance spectroscopy. The results reveal that all annealed films crystallize and exhibit good cycle performance. The optimum annealing temperature is about 700 °C. The steady-state discharge capacity of the films is about 157 mAh g⁻¹ at a medium discharge/charge current density of 10 μA cm⁻². At a considerably higher discharge/charge current density of 60 μA cm⁻² (about 3.45 C) the discharge capacity of the films remains steady at a relative high value (146 mAh g⁻¹). The cycleability of the films is excellent. This implies that such films are suitable for electrodes to be used at high discharge/charge current density.     

Pd/Ag alloy as an application for hydrogen sensing

A highly sensitive H₂ gas sensor was fabricated using a Micro Electromechanical Systems (MEMS) procedure having an embedded micro-heater. The palladium-silver (Pd/Ag having stoichiometric ratios 77:23) thin film was deposited by the RF/DC magnetron sputtering and used as the hydrogen sensing layer designed as a zig-zag pattern. Morphological and structural properties of the Pd/Ag thin film was studied by Field emission scanning electron microscope (FESEM), Atomic force microscopy (AFM) and Energy Dispersive Analysis of X-rays respectively. The working temperature of the micro heater showed a linear relation with variations of the heater voltage. The electro thermal properties of the H₂ sensor were studied by finite element method (FEM). The sensing properties of the fabricated H₂ sensor as the change of electrical resistance were studied with respect to hydrogen concentration and temperature. Experimental results showed high sensor response and response time after application of the heater voltage. The sensing properties of the alloyed Pd/Ag thin film were more improved than those of pure palladium. The maximum sensor response (R_s) of the fabricated H₂ sensor was 14.26% for 1000 ppm H₂. The sensor response of the fabricated H₂ sensor showed linear behavior with the heater voltage (operating temperature) and positively corresponded with the hydrogen concentration.     

Fabrication of LiCoO2 cathode powder for thin film battery by aerosol flame deposition

Crystalline LiCoO₂ nano-particles for thin film battery were synthesized and deposited by aerosol flame deposition (AFD). The aqueous precursor solution of the lithium nitrate and cobalt acetate was atomized with an ultrasonic vibrator and subsequently carried into the central tube of the torch by flowing dry Ar gas. LiCoO₂ were formed by oxy-hydrogen flame and deposited on a substrate placed in a heating stage. The deposited soot film composed of nano-sized particles was subsequently consolidated into a dense film by high temperature heat treatment at 500–800 °C for 5 h and characterized by SEM, XRD, and Raman spectroscopy. The crystalline carbonates and oxide were first formed by the deposition and the subsequent heat treatment converted those to LiCoO₂. The FWHMs of the XRD peaks were reduced and their intensity increased as the heat treatment temperature increased, which is due to improved crystallinity. When judged from the low enough cation mixing and well-developed layered structure, it is believed that the LiCoO₂ film satisfied the quality standard for the real application. SEM measurements showed that LiCoO₂ were nano-crystalline structure with the average particle size <70 nm and the particle size increased with the increase of heat treatment temperature. The thickness of thin film LiCoO₂ before the consolidation process was about 15 μm and reduced to about 4 μm after sintering.     

High performance hydrogen sensor based on Pd/TiO2 composite film

Hydrogen sensors with fast response and recovery rate based on nanoporous palladium (Pd) and titanium dioxide (TiO₂) composite films supported by anodic aluminum oxide (AAO) template have been demonstrated. Nanoporous TiO₂ film was sprayed on the porous AAO templates, followed by Pd film deposited on TiO₂ layer by DC magnetron sputtering. We have researched the detection performance of the hydrogen sensors depending on different thickness of TiO₂ layer from 6 to 30 nm with keeping the thickness of Pd as 30 nm. The results have demonstrated the sensors with 10 nm thickness of TiO₂ achieve the best performance with a response/recovery time as short as 4/8s at 0.8% and 0.4% hydrogen concentration, respectively. The sensors exhibited very good performance under hydrogen concentrations from 0.4% to 1.8%.     

Hydrogen sensing by plasmon decoupling effect in nanostructured Pd/Au films

Plasmon coupling effect occurs in plasmonic nanostructures when interparticle distances are in the order of particle size leading to spectral shifts in the plasmonic band. This effect has been recently highlighted for measurement of fluctuations in the interparticle distance at nanoscale level. In this study, nanostructured thin Au films were deposited on quartz substrates by pulsed laser deposition (PLD) for sensing of hydrogen gas. A blue shift from 730 to 560 nm in LSPR of Au films was observed when substrate temperatures rises from 25 to 600 °C due to variation in morphology of films from a continuous surface composed of tiny agglomerates to granular surface composed of bigger particles with increased interparticle spacing. For plasmon coupling sensing of hydrogen, a thin Pd film was deposited on top of nanostructured Au films. Upon hydrogen exposure, up to12 nm blue shift within few seconds was observed depending on hydrogen concentration. Based on field emission scanning electron microscope (FESEM) images and finite-difference time-domain (FDTD) simulations, this plasmon sensing is explained by hydrogen-induced decoupling due to the formation of surface stresses in Pd, which can affect the LSPR via an increase in interparticle spacing of Au nanoislands.     

Photoelectron spectroscopy and atomic force microscopy study of 1,2-dicyano-methanofullerene C60(CN)2 thin film for photovoltaic applications

1, 2-dicyano-methanofullerene (C₆₀(CN)₂) is a soluble fullerene derivative that has been reported to have stronger electron affinity than parent C₆₀. Ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) experiments were carried out on C₆₀(CN)₂ thin films spin coated on heavily doped n-type Si substrate. UPS spectra enabled the determination of the vacuum shift at the fullerene derivative/Si interface and the onset of the highest occupied molecular orbital (HOMO). From the UV-vis absorption spectra of C₆₀(CN)₂ thin films spin coated on quartz substrates, the optical band gap (E_g) and the onset of absorption were determined. These measurements allowed the determination of the lowest occupied molecular orbital (LUMO) position. The morphology of the deposited film was probed by AFM and reveals non-uniformity of the thin film. Open circuit voltage (V_oc) measurements on P3HT/C₆₀(CN)₂ based organic solar cell device are compared to the commonly used P3HT/PCBM device.